1. Field of the Invention
The present invention relates generally to improvements in optical communication systems. More particularly, the present invention relates to improvements in optical amplifier wavelength shifters.
2. Description of Prior Art
In optical communication systems, it is often desirable to shift an optical signal from one wavelength to another. For example, wavelength shifting can significantly increase total system capacity of wavelength division multiplexed (WDM) optical systems, which simultaneously utilize several optical carrier signal wavelengths modulated by different data streams. A large high-capacity WDM optical network may therefore be constructed by interconnecting smaller WDM optical networks through wavelength shifters. The same set of carrier signal wavelengths are used within each of the smaller networks, while wavelength shifting of the optical carrier signals passing between the smaller networks allows frequency reuse of the available optical bandwidth and thereby increased system capacity. See U.S. patent application Ser. No. 07/880,728 entitled "Optical Wavelength Shifter" and assigned to the assignee of the present invention.
As another example, optical signals may be wavelength shifted to overcome source bandwidth limitations which restrict the range of possible carrier signal wavelengths. A wavelength shifter permits use of carrier signals with wavelengths outside of the optical source tuning range by shifting a modulated optical carrier signal from a first wavelength to a second wavelength. The first wavelength is generally the output carrier signal from the optical source, while the second wavelength may be any other desired wavelength within the operating bandwidth of the wavelength shifter. Wavelength shifting is also important in other optical applications, including optical switching, optical logic elements, signal regeneration, time demultiplexing and optical taps.
Wavelength shifting, also commonly referred to as wavelength or frequency conversion, may be carried out using a number of available techniques. One such technique is optical four-photon mixing. Four-photon mixing is a nonlinear optical process which produces mixing products at different wavelengths by mixing an input optical signal with one or more higher power optical signals, or pump signals, in a nonlinear mixing medium such as a semiconductor laser amplifier or a length of dispersion shifted optical fiber. Four-photon mixing can be used in principle to wavelength shift signals at bit rates of up to about 100 Gbits/sec or more, and has been demonstrated at bit rates of about 10 Gbits/s. However, four-photon mixing suffers from a number of drawbacks, including dependence of mixing efficiency on relative input and pump signal polarization, sensitivity to input and pump signal power and wavelength, possible output signal interference from undesired mixing products and low conversion efficiency.
Another known technique uses semiconductor optical amplifier gain compression to produce the complement of intensity modulation of a first optical signal at a given wavelength on an unmodulated continuous wave (CW) second optical signal at a different wavelength. The first optical signal is often referred to as the pump signal, while the unmodulated second signal is referred to as the probe signal. The intensity modulation on the pump signal varies the optical amplifier gain and in effect modulates the amplification of the previously unmodulated probe signal, such that the complement of the intensity modulation on the pump signal also appears on the amplified probe signal. See B. Glance et al., "High Performance Optical Wavelength Shifter", Electronics Letters, Vol. 28, No. 18, Aug. 27, 1992. Wavelength shifting via optical amplifier gain compression provides a number of advantages. These include potential polarization independence, reduced sensitivity to pump and probe signal parameters such as power level and wavelength, and conversion gain. See J. Wiesenfeld and B. Glance, "Cascadability and Fanout of Semiconductor Optical Amplifier Wavelength Shifter," IEEE Photonics Technology Letters, Vol 4, No. 10, October 1992.
Presently available gain compression wavelength shifting techniques typically utilize a probe signal with a low power relative to the intensity-modulated pump signal. A relatively low power probe signal is used because it has been recognized that a degradation in the wavelength shifter output extinction ratio may result if the probe signal power is increased to a point at which it begins to saturate the amplifier gain. The extinction ratio, also known as contrast ratio, refers to the ratio of power corresponding to a logic level one, or "mark power", to power corresponding to a logic level zero, or "space power", in the data stream of the wavelength shifted signal. The extinction ratio is reduced by an increase in probe signal power because the fluctuation in amplifier gain resulting from the intensity modulation of the pump signal is reduced in the presence of a higher power gain-compressing probe signal. A probe power level of about -15.0 dBm or less at the amplifier input facet (dBm refers to dB relative to 1.0 mW) is therefore typically used for the probe signal. A maximum bit rate of about 4.0 Gbits/s has been demonstrated using a probe signal at a power level in this range. See C. Joergensen et al., "4 Gb/s Optical Wavelength Conversion Using Semiconductor Optical Amplifiers", IEEE Photonics Technology Letters, Vol.5, No.6, June 1993.
The achievable bit rate in gain compression wavelength shifters is limited by the gain recovery time, or carrier lifetime, of the optical amplifier. In an optical amplifier, the gain recovery time is dependent, in part, on the amplifier internal photon density. When the pump signal makes a transition from low to high power, the high pump power saturates the amplifier, reducing the gain of the amplifier for the probe signal but producing a high internal photon density. The high photon density leads to a rapid gain recovery time, and a fast probe signal transition, or fall time, from high to low amplification. However, when the pump signal makes a transition from high to low power, the internal photon density in the amplifier is greatly reduced, leading to a slower gain recovery time, and hence a slow probe signal transition, or rise time, from low to high amplification. The rise time of the intensity modulation reproduced on the probe signal is therefore slow, and limits the maximum speed or bit rate capacity of the wavelength shifter. For example, the minimum achievable output rise times are presently about 200 to 250 picoseconds for InGaAsP semiconductor optical amplifiers operating near 1.5 .mu.m, resulting in a maximum bit rate of about 4.0 Gbits/sec for these devices. In order to increase the capacity of optical systems using wavelength shifters, further reductions in output signal rise times are needed.
Although it may be known that an increase in the internal photon density in a semiconductor laser or amplifier will lead to a shorter carrier lifetime, it has not heretofore been considered acceptable to achieve such an increase in photon density by increasing the probe signal input power. Other techniques for increasing internal photon density have been suggested, including the use of specially designed optical amplifiers with large facet reflectivities. For example, in the C. Joergensen et al. article, cited above, an increase in internal photon density is achieved by increasing the facet reflectivity from 5.0.times.10.sup.-4 to 0.36. The increased reflectivity resulted in a decrease in probe signal modulation rise time from 175 to 60 picoseconds at an optical amplifier bias current of about 70 mA. However, increased reflectivity also produces significant gain ripple and wavelength sensitivity, and is therefore generally not considered a suitable technique for increasing the bit rate capacity of practical broadband wavelength shifters.
As is apparent from the above, a need exists for a method and apparatus which permit higher bit rate optical amplifier wavelength shifting over a broad bandwidth, without significant performance degradation or additional hardware design and implementation costs.